A motor control center, also referred to as an MCC, is generally a cabinet-like structure that is divided into modular sections designed to accept a plurality of component motor control center units, also referred to as MCC units. An MCC unit is generally a drawer-like container of electrical components, such as circuit breakers, motor starters, relays, power meters, programmable logic controllers, and other control circuit components used for electrical power distribution in industrial and commercial applications. The MCC unit slides into an MCC section and plugs into a common power bus running vertically along the back side of the MCC section. The MCC unit includes a connector, also referred to as a stab assembly or stabs, arranged on the back side of the unit, to plug into the power bus. When an MCC unit is installed, serviced, or removed from an MCC section, it is of utmost importance to prevent the operator from being exposed to electrical hazards. In existing MCC units, an operator accessible unit on/off handle on the front of the MCC unit enables a circuit breaker within the unit to be switched off before the MCC unit is installed in or removed from the MCC section.
Unit interlock mechanisms have been provided in the past to prevent inserting or removing MCC units from an MCC section, without first actuating an advance/retract handle located on the front of the MCC unit, to retract the stabs of the unit to prevent contact with the power bus. The unit interlock blocks inserting the MCC unit into the motor control center section, without first actuating the advance/retract handle for the advance/retract mechanism to retract the stabs to prevent contact with a power bus. To insert the MCC unit into the MCC section, the operator slides the MCC unit into an available opening in the section, so that the back wall of the MCC unit is close to the common power bus running along the back side of the section. The operator then actuates the advance/retract handle for the advance/retract mechanism to connect the stabs of the MCC unit to the power bus.
Many MCC units include electrical devices such as circuit breakers connected to the stabs that may be any form of connector of the circuit breaker to the power bus. One example of an existing MCC unit mounts the circuit breaker and the stabs on a pivoted platform, also referred to as the unit line power jaws. The pivoted platform is connected to the advance/retract mechanism that is operated by the advance/retract handle. Operating the advance/retract handle forces the platform to pivot and either advance the stabs to contact the power bus or retract the stabs to disconnect them from the power bus. The existing unit interlock in an MCC unit, blocks the unit from being inserted into the MCC section when the stabs are advanced, to avoid any connection to the power bus during insertion of the unit. After the MCC unit has been inserted into the MCC section, the unit interlock blocks the unit from being removed from the MCC section after stabs have been advanced and connected to the power bus. The advance/retract mechanism must retract the stabs before the unit interlock allows the unit to be removed from the section.